The present invention relates to an apparatus and method for purifying boron trichloride by removal of phosgene in which the phosgene is decomposed by exposure to electromagnetic radiation of ultra-violet wavelength. More particularly, the present invention relates to such an apparatus and method in which the equilibrium of the decomposition components of the phosgene within the boron trichloride is disturbed to remove the decomposition components from the solution.
Boron trichloride (BCl.sub.3) has varied uses in many industrial processes. Although the purity of boron trichloride is of importance in most applications, it is particularly important in the field of micro-electronics. The use of boron trichloride in the electronics industry generally requires impurity levels of phosgene (COCl.sub.2) in an amount ranging from between about 10 and about 0.5 parts per million. It is difficult to remove phosgene from boron trichloride through distillation or other common separation techniques due to the similarity of the volatilities of boron trichloride and phosgene and also due to the similarity of the molecular structures.
As a result, alternative methods of phosgene removal have been proposed that involve the exposure of the boron trichloride to electromagnetic radiation. For instance, in U.S. Pat. No. 4,063,896, boron trichloride is exposed to CO.sub.2 laser radiation passed through a laser cell. The radiation causes the phosgene to disassociate into dissociation products such as carbon monoxide and chlorine. In U.S. Pat. No. 4,405,423, boron trichloride is dissolved in a cryogenic solution and exposed to an ultra-violet light emitted from a mercury arc lamp to reduce the phosgene impurity.
The problem with such methods (described in the above-referenced patents) is that if the dissociation products of phosgene are not removed from solution with boron trichloride, they will recombine to re-produce the phosgene impurity. As the dissociation products of phosgene build-up over time, the recombination rate increases and eventually the recombination rate equals the dissociation rate and a state of equilibrium is reached between the phosgene and its dissociation products. Once equilibrium is reached, there is no further reduction in the phosgene concentration. Thus, these methods are not effective to reduce impurity levels below a certain point and thus, cannot be used to reduce impurity levels of phosgene to that required for the electronics industry.
As will be discussed, the present invention provides an inexpensive method of treating the solution of boron trichloride and decomposition products of phosgene to effect removal of the dissociation products and thus allow production of boron trichloride at a very low level of phosgene impurity.